This invention relates to dielectric members and, in particular, to dielectric members used for electrical isolation in liquid electrolyte fuel cells.
In a carbonate (liquid electrolyte) fuel cell stack which uses external manifolds for gas supply and discharge, the manifolds are separated from the fuel cell stack by a ceramic dielectric member in the form of a picture frame. This is described in U.S. Pat. No. 4,414,294, in which a ceramic dielectric frame is provided for electrical insulation and reactant gas sealing.
The voltage across the carbonate fuel stack is normally between 100 to as high as 1000 volts depending on the number of cells in a stack and the electrical configuration of the stacks when arranged in a power plant. The ceramic dielectric frame itself is capable of providing electrical isolation to thousands of volts. However, during fuel cell operation, the liquid electrolyte in the fuel cell stack tends to creep over the surface of the dielectric frame. For molten carbonate fuel cells, the electrolyte is alkali carbonates. At the operating temperature, the carbonates are liquid. Once the electrolyte contacts the frame, a thin continuous layer of conductive liquid electrolyte film forms on the frame surfaces. Consequently, the dielectric characteristic of the frame is reduced.
This compromises the integrity of the fuel cell stack. As a result, techniques are being sought for preventing or reducing the electrolyte creepage. One proposed technique is to situate a layer of porous gasket between the dielectric frame and the stack. This gasket enhances the gas sealing and helps retard electrolyte flow from the stack. A gasket of this type is described in U.S. Pat. No. 5,100,692.
The gasket of the ""692 patent offers one way of retarding electrolyte creepage along the surface of a ceramic dielectric frame. Other techniques for reducing electrolyte creepage are still being sought.
It is, therefore, an object of the present invention to provide a dielectric member which exhibits reduced surface creepage of fuel cell electrolyte when used in a fuel cell stack.
It is a further object of the present invention to provide a dielectric member in which such reduced creepage of fuel cell electrolyte is realized simply and at low cost.
In accordance with the principles of the present invention, the above and other objectives are realized in a dielectric member for electrically insulating a manifold or other component from a fuel cell stack by forming the member to have a surface roughness of Ra (Ra: the average deviation of the profile from the mean line, in xcexc inches) of 10 or less. A surface having this degree of roughness can be realized by lapping or polishing the dielectric member or by using a fine grained powder to form the member. A preferable dielectric material for forming the member is Al2O3 with a density higher than 95% of the theoretical density. A more preferable density for the Al2O3 is a density higher than 98% of the theoretical density.